Electrical energy storage units are used for storing and preparing electrical energy in many application areas such as, for example, in motor vehicles, rail vehicles, cranes, or with decentralized power supply systems. The energy storage unit has at least one energy storage cell, which is often based on an electrochemical or purely electrical storage concept. A plurality of energy storage cells are connected in series in order to increase the electrical voltage provided by the storage cell or the available storage capacity.
For example, a plurality of double layer condensers connected in series form an energy storage stack in order to be able to furnish a higher output voltage, since the maximum available output voltage of an individual double layer condenser is generally limited to a small voltage range between 2.5 V and 2.7 V. If an application requires a nominal voltage of 100 V, 40 cells each having 2.5 V are collected into a single condenser stack.
Due to the high manufacturing tolerances, the storage cells have different capacitances and internal series resistances, as well as various self-discharging characteristics. This leads to non-uniform cell voltages within a condenser stack, which can give rise to an excess voltage in cells with low capacitance and to polarity reversal in cells with higher capacitance. The different cell voltages also give rise to an irregular change in the individual cells. A process for voltage equalization between the cells is thereby sought as a preventative measure.
The problem described previously occurs in principle in all energy storage stacks, independent of the storage concept used for the individual cells, i.e., electrochemical or purely electrical storage concepts.
Devices and methods for balancing the cell voltages are known from the prior art, which alongside trivial passive balancing procedures like parallel balancing resistors or Zener diodes, also suggest a charge transfer condenser that enables a transfer of charge between two cells having different cell voltages.
The object of the present disclosure is to continue the development of devices known from the prior art, in order to ensure a simplified and more effective monitoring and voltage balancing capability of the cell voltages.
This objective is achieved by a device for monitoring and balancing the cell voltages of at least two electrical energy storage cells of a multi-cell energy storage stack.
In one example, a voltage measuring unit is provided to monitor the cell voltage of the at least two electrical energy storage cells connected in series, which is connected directly in an electrically conducting manner through a first combinatorial circuit to each energy storage cell. An energy storage element is furthermore provided that acts as the energy transfer medium between at least two energy storage cells that are electrically connected in series. The energy storage medium can be directly connected electrically by a second combinatorial circuit through the first combinatorial circuit to each energy storage cell of the multi-cell energy storage stack. A control unit is furthermore provided that is connected to the voltage measuring unit and controls the first and second combinatorial circuits by taking into consideration the values transmitted by the voltage measuring unit.
In one embodiment, the voltage measuring unit can be selectively switched in parallel by the first combinatorial circuit to each individual energy storage cell of the energy storage stack for monitoring and/or measuring the voltage. A single voltage measuring unit is thereby sufficient to convert a voltage observation and/or measurement for each individual energy storage cell of the energy storage stack.
The energy storage element can be selectively switched in parallel to each energy storage cell by the first and second combinatorial circuit. The energy storage element may also operate as an energy transfer element, which transfers a portion of the electrical charge from an arbitrary cell to another arbitrary cell once or a plurality of times.
It has proven to be especially advantageous to use a charge transfer condenser as the energy storage element, and in particular a double layer condenser. If the charge transfer condenser is connected in parallel to an energy storage cell, an applied potential difference between the charge transfer condenser and energy storage cell then produces a charging or discharging process of the charge transfer condenser. A parallel connection of the charge transfer condenser with an energy storage cell at higher potentials causes a discharge of the cell in favor of the charge transfer condenser, until an equalization of the potential exists between the components that are connected in parallel. A parallel connection of the charge transfer condenser with an energy storage cell at lower potentials causes a discharge of the charge transfer condenser in favor of the storage cell, until an equalization of the potential exists between the components that are connected in parallel.
Both combinatorial circuits are advantageously independently controllable by the control unit. The topology of the first and second combinatorial circuits is developed so that it is possible to switch selectively in parallel from the charge transfer condenser to any cell.
The design of the individual switching elements of the first and/or second combinatorial circuits can be selected in a fundamentally arbitrary manner. Electromechanical relays are low loss devices and exhibit almost no voltage drop, but they have too small a lifetime. Bipolar transistors and IGBTs produce a disadvantageous high voltage drop in the circuit. The present disclosure makes use of semiconductor switches as the switching elements inside the first and/or second combinatorial circuit. The semiconductor switches used here produce only an extremely small voltage drop in the circuit, and are also low loss devices.
In an especially advantageous embodiment, MOSFET switches are used as semiconductor switches. Under certain circumstances the diode integrated in parallel in the MOSFET short circuits all the cells of the energy storage stack. For this reason a switching element of the first and/or second combinatorial circuit includes at least one antiserial protective circuit consisting of two MOSFET switches.
Unfavorable spurious common mode voltages can be evoked by the parallel connection of the voltage measuring unit having at least one arbitrarily selected energy storage cell, which lead to corruption of the measured results. The voltage measuring unit thus advantageously includes at least one precision capacitor and at least two switching elements for selectively decoupling the voltage measuring unit from the energy storage cell to be measured. the precision capacitor is connected to the input of the voltage measuring unit and is charged during the measurement phase through the first combinatorial circuit to the applied cell voltage of the energy storage cell to be measured, while the integrated measurement amplifier is decoupled by the at least two switching elements of the voltage measuring unit. As soon as the measurement amplifier holds the cell voltage, the connection to the measurement amplifier is terminated by element of the switching elements.
In a possible embodiment of the present disclosure at least one energy storage cell of the energy storage stack is an electrochemical accumulator or a condenser, in particular a double layer condenser. The individual energy storage cells can be the same or different. The energy storage element is advantageously chosen to be of the same type. The capacitance of the energy storage element advantageously corresponds approximately to the tolerance in the capacitance of the individual cells that are used. This enables an effective and rapid charge transfer between the energy storage cells.
It can be provided that the control unit has at least one data storage unit for storing one or more voltage measuring values detected by the voltage measuring unit. In order to evaluate the measured voltage value that is detected, it can be advantageous when one or a plurality of reference voltage values is stored in the data storage unit. It is also possible to store one or a plurality of voltage tolerance values that define a permissible deviation corridor for the measured voltage values.
In order to integrate or link the device according to the present disclosure to a subordinate or higher-ranking system, the control unit includes at least one data interface for data communications. The data interface serves to exchange the measured voltage values that are measured or more extensively analyzed values or data relevant for control purposes. The data interface serves in particular to link the device to an existing system data bus.
The present disclosure furthermore relates to a method for monitoring and balancing the cell voltages of at least two energy storage cells of a multi-cell energy storage stack that are electrically connected in series, wherein the method according to the present disclosure is implemented on a device according to one of the previously described variants of the device according to the present disclosure. The method may have the same advantageous properties as the device according to the present disclosure.
The method presented here relates to the control of the first and second combinatorial circuits by the control unit provided, in order to assure a selective measurement of the individual cell voltages and to carry out a voltage balancing operation between two or more energy cells as a function of the measurement value detected.
The control unit may control the first and second combinatorial circuit, in order to measure and/or monitor all cell voltages of the energy storage stack selectively or in a specified sequence by element of the voltage measuring device.
In particular, an evaluation of the measured voltage values is carried out to determine the voltage deviation of the first measured voltage value from one or more stored reference voltages. A deviation of the measured voltage values from one or more reference voltage values signals a critical operating condition of the energy storage stack.
An advantage of the method according to the present disclosure is that the control unit first of all identifies the energy storage cells of the energy storage stack with minimal and maximal cell voltages. The energy storage cells are balanced by using the energy storage element, up to the point where the voltage difference of both of the measured cell voltages falls below a predetermined reference value. This operational step is carried out recursively, up to the point where the maximum cell voltage difference occurring between two arbitrarily selected energy storage cells falls below a predetermined reference value.
An additional advantage of the method according to the present disclosure is that the control unit, with the aid of the measured voltage values, yield the charge energy state of each energy storage cell, and consequently the charge state of the entire energy storage stack.
It is also possible that the control unit with the aid of the measured voltage value provides an inference with respect to the relative state of aging, at least of a single energy storage cell.
The present disclosure furthermore relates to the use of such a device according to the present disclosure or the method according to the present disclosure in a machine having a multi-cell energy storage stack, such as e.g. in hybrid motor vehicles, rail vehicles, container cranes, and hybrid power shovels.
Additional advantages and details of the present disclosure will be explained with the aid of the embodiments shown in greater detail in the drawings.